SUNRPC: use sv_lock to protect updates to sv_nrthreads.

Using sv_lock means we don't need to hold the service mutex over these
updates.

In particular,  svc_exit_thread() no longer requires synchronisation, so
threads can exit asynchronously.

Note that we could use an atomic_t, but as there are many more read
sites than writes, that would add unnecessary noise to the code.
Some reads are already racy, and there is no need for them to not be.
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfssvc.c

@@ -55,9 +55,8 @@ static __be32			nfsd_init_request(struct svc_rqst *,
 						struct svc_process_info *);
 
 /*
- * nfsd_mutex protects nn->nfsd_serv -- both the pointer itself and the members
- * of the svc_serv struct. In particular, ->sv_nrthreads but also to some
- * extent ->sv_temp_socks and ->sv_permsocks.
+ * nfsd_mutex protects nn->nfsd_serv -- both the pointer itself and some members
+ * of the svc_serv struct such as ->sv_temp_socks and ->sv_permsocks.
  *
  * If (out side the lock) nn->nfsd_serv is non-NULL, then it must point to a
  * properly initialised 'struct svc_serv' with ->sv_nrthreads > 0 (unless

net/sunrpc/svc.c

@@ -523,7 +523,7 @@ EXPORT_SYMBOL_GPL(svc_shutdown_net);
 
 /*
  * Destroy an RPC service. Should be called with appropriate locking to
- * protect the sv_nrthreads, sv_permsocks and sv_tempsocks.
+ * protect sv_permsocks and sv_tempsocks.
  */
 void
 svc_destroy(struct kref *ref)
@@ -639,7 +639,10 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
 		return ERR_PTR(-ENOMEM);
 
 	svc_get(serv);
-	serv->sv_nrthreads++;
+	spin_lock_bh(&serv->sv_lock);
+	serv->sv_nrthreads += 1;
+	spin_unlock_bh(&serv->sv_lock);
+
 	spin_lock_bh(&pool->sp_lock);
 	pool->sp_nrthreads++;
 	list_add_rcu(&rqstp->rq_all, &pool->sp_all_threads);
@@ -880,7 +883,9 @@ svc_exit_thread(struct svc_rqst *rqstp)
 		list_del_rcu(&rqstp->rq_all);
 	spin_unlock_bh(&pool->sp_lock);
 
+	spin_lock_bh(&serv->sv_lock);
 	serv->sv_nrthreads -= 1;
+	spin_unlock_bh(&serv->sv_lock);
 	svc_sock_update_bufs(serv);
 
 	svc_rqst_free(rqstp);